1. Field of the Invention
The present invention relates to adhesive compositions based on polychloroprene dispersions having improved adhesive properties, to a process for the preparation thereof, and to their use as a contact adhesive for inorganic or organic substrates.
2. Description of the Prior Art
The preparation of polychloroprene by emulsion polymerization in an aqueous-alkaline medium is known and described in xe2x80x9cUllmanns Encyclopxc3xa4die der technischen Chemiexe2x80x9d, Volume 9, p. 366, Verlag Urban und Schwarzenberg, Munich-Berlin 1957; xe2x80x9cEncyclopedia of Polymer Science and Technologyxe2x80x9d, Vol. 3, p. 705-730, John Wiley, New York 1965; and xe2x80x9cMethoden der Organischen Chemiexe2x80x9d (Houben-Weyl) XIV/1, 738 et seq. Georg Thieme Verlag Stuttgart 1961.
Suitable emulsifiers include, in principle, all compounds and mixtures thereof that stabilize the emulsion sufficiently, such as water-soluble salts, especially sodium, potassium and ammonium salts, of long-chained fatty acids, colophony and colophony derivatives; higher molecular weight alcohol sulfates; arylsulfonic acids; formaldehyde condensation products of arylsulfonic acids; non-ionic emulsifiers based on polyethylene oxide and on polypropylene oxide; and polymers having an emulsifying action, such as polyvinyl alcohol (DE-A 2 307 81 1, DE-A 2 426 012, DE-A 2 514 666, DE-A 2 527 320, DE-A 2 755 074, DE-A 3 246 748, DE-A 1 271 405, 1 301 502, U.S. Pat. No. 2,234,215, JP-A 60-31 510 (=58-136 824 of 28, Jul. 1983)).
Polychloroprene is used either, after appropriate compounding and vulcanization, in the manufacture of technical rubber articles, or it is used as a raw material for contact adhesives (xe2x80x9cHandbook of adhesivesxe2x80x9d, Chapter 21, Verlag Van Nostrand Reinhold, New York, 2nd edition 1977).
Polychloroprene-based contact adhesives are predominantly solvent-based adhesives that are applied to the two parts to be bonded and allowed to dry. By subsequently joining the two parts under pressure, a bond having high strength is obtained. In the case of solvent-based contact adhesives, the long period of time within which bonding is possible at room temperature (xe2x80x9copen timexe2x80x9d) is regarded as particularly advantageous.
For ecological and economic reasons there is a growing need for suitable aqueous polychloroprene dispersions that can be processed to corresponding aqueous adhesive formulations. However, such formulations have the disadvantage that, after evaporation of the water, the xe2x80x9copen timexe2x80x9dxe2x80x94in comparison with solvent-based adhesivesxe2x80x94is markedly shorter. In the case in particular of the addition of high-melting resins, which increase the resistance of the bonds to heat, readily reproducible contacting of the substrates to be bonded, especially in the case of substrates that are not very absorbent or are non-absorbent, can be achieved only by previous heat activation of the dry adhesive film (H. W. Lucas in xe2x80x9cAdhesives Agexe2x80x9d 1992, Volume 2, p. 28).
In the present application, xe2x80x9copen timexe2x80x9d is defined as the interval between the time at which the applied adhesive is sufficiently dry to permit bonding of the substrates and the time at which it is no longer possible to achieve a satisfactory bond at room temperature simply by applying pressure.
Accordingly, it is an object of the present invention to provide an aqueous adhesive composition based on an aqueous polychloroprene dispersion, in which the composition, after evaporation of the water, possesses a long xe2x80x9copen timexe2x80x9d during which bonding is possible at room temperature without heat activation. It is an additional object of the present invention to obtain an improvement in the heat resistance of the contact adhesive.
The first object can be achieved with the aqueous adhesive compositions based on polychloroprene dispersions, which can be obtained by the continuous polymerization of chloroprene in aqueous emulsion in the presence of unmodified resin acids as emulsifiers. The aqueous dispersions of adhesive raw materials can then be processed with known adhesive additives to form adhesive compositions that have a sufficiently long xe2x80x9copen timexe2x80x9d that bonding is possible at room temperature without heat activation. The second object can be achieved by adding a high-melting resin to the adhesive composition.
The present invention relates to adhesive compositions containing a polychloroprene dispersion containing a tricyclic diterpenecarboxylic acid having at least two conjugated Cxe2x95x90C double bonds per molecule.
The present invention also relates to a process for preparing these adhesive compositions and to their use as contact adhesives for organic and inorganic substrates.